I initially didn't understand what was the purpose of the shorcut, as nothing was happening. After some exchanges on the site, Martin Z suggested to check whether the menu option (View | Navigation | Zoom Camera 1:1) did show the same problem. It didn't, the menu option worked as expected.
Confirm the problem is with the shortcut
This is an indication of a potential keyboard problem, and for those suspecting something similar:
Confirm the shortcut is really the one you think. You may look at the menu entry or use space to display the search window. The shortcut will be indicated near the description of the function.
If the shortcut is the one expected, then try running the function using the menu. If it doesn't work better, then the problem is elsewhere.
Keyboard problem confirmed... what is the cause?
In my case I use a laptop and an external USB keypad. The problem is the keypad, which brand is Targus:
I don't know whether this device doesn't generate the correct key code for key Enter, or whether Blender developers didn't bother or weren't able to differentiate the internal keyboard Enter from the USB keypad Enter, but in Blender API both keys appear the same.
The solution is to change the shortcut associated with the function. It's done in user preferences (open this window with the combination ctrlaltU (or menu entry File | User Preferences in the "Info" header).
Select Input tab and type a part of the function name, for instance "1:1" in my case, in the search field to display the function and its binding:
- Put the mouse pointer over the keyboard shortcut
- Press Enter to activate the key recorder
- Type a new shortcut, in my case crtlaltreturn to prevent overwriting one already in use.
Don't forget to save the user preferences, else the new binding will be lost when Blender will be closed.